ENAMELS FOR NEW ENAMELLING PROCESS
Lyudmyla Bragina - Technical University "KPI", Ukraine
Andrey Bondarenko - Trade House "Emal'zavod", Ukraine

Abstract
The connection between conductivity and corrosion activity of enamel melts and the mechanism of their interaction with metal substrates has been established. The role enamel melts conductivity on adherence has been defined. The features of frit properties for dry electrostatic spraying on steel have been discussed. Industrial compositions of non-cobalt ground coats for Puesta and Combismalt have been produced.

Introduction
The technology "Two coats / one fire" is widely used for electrostatic (1,2) and electrical electrophoretic (3) coatings application on steel, especially in production of gas and appliances. The "One coats / one fire" technology is widely used for microwaves (4) and boilers (5).
Recently "2C/1F" is becoming widely used for cast iron enamelling (6). There is some progress in the usage of 2C/1F technology in case of "Combismalt" (7).
In the former USSR it was as early as the fifties that industrial implementation of electrostatic spraying of slips on cast-iron baths was accomplished by Lugansk Enamel Plant (8). Both the technology and the equipment were developed by M. Livshitz in the Scientific research institute for sanitary ware enamelling (Moscow, Russia). In the field of electrostatic spraying of enamel powders one should note the work of Y. Litvin (9) and A. Sarukhanishvili (10). Electrostatic spraying of enamel coatings on components of gas stoves is carried out by Brest (Belarus) and on washing machine components in Kirov (Russia).
For the first time in the Ukraine, dry electrostatic spraying was used for enamelling gas and electrical stoves by the "Azovmash" concern, along with Mariupol and the Kharkov State Politechnic University, Ukraine, in 1993 with the use of technological line "Estap" (Slovakia).
However in this field, as in the whole of enamelling, there are several problems to be addressed. Synthesis of important new ground coat enamels without traditional adherence oxides; increasing of powder adhesion with simultaneous lowering of the electric field voltage.
Provision of same coatings thickness on inside surface of articles with complicated configuration, for example, boilers.
Establishment of formation process and adherence with substrate of the twolayer coating process during the single fire. Control of slip rheology ("Combismalt") and ground and cover enamel melt rheology; elimination of defects which are typical for "2 C / 1 F" et al. The paper attempts to solve several of the above.

Results and discussion
Development of non-cobalt ground enamels
The major requirement of ground and direct enamels for advanced modern enamelling technologies is for a high adherence at relatively low firing temperature. A substitution of traditional adhesion activators - cobalt and nickel oxides - by cheaper substances, for example copper and manganese oxides et al. is possible.
For electrostatic application, low melting ground and cover enamels must have high volume resistivity ρv under normal conditions. The requirements for wetting ability, viscosity and surface tension were taken into account in the synthesis of ground coat enamels for "2C/1F", "Puesta" and "Combismalt". Earlier during the development of an electrochemical adhesion theory, research was carried out in cooperation with Professor Svirskiy L.D. The aim of the research was to establish a connection between the adherence of ground coat enamels, electrical conductivity and the ground coat melt corrosion action on low carbon steel; oxygen quantity in the furnace atmosphere and the oxygen diffusion rate through the melt, and the reactions at this metal-coating interface.
These investigations were made by prominent scientists. The uniqueness of our work was in establishing the role of conductivity in groundcoat enamel melts which contained different adhesion promotors, including copper oxide. The conductivity of melts was measured at temperature intervals from 600-1000 °C, by the compensation method, using an alternating current bridge, quartz U-form cell and platinum electrodes. The corrosion action of melts was measured on steel bars made from low carbon steel 08Π diameter by the change after a 5-hour exposure in a melt at 1000 °C, on steel oxidation by a thermogravimetric method, by oxygen diffusion through the melt, and the adherence of coatings and interface relief coating-substrate were measured by standard methods. For the experiments a common purpose ground coat enamel for steel was selected, (%): SiO2 - 50; Al₂O₃ - 8; B₂O₃ - 15; Na₂O - 22; CaF₂ - 5.
In that enamel cobalt, nickel and copper oxides in quantity from 0,0006 to 0,09 g- mol/100 weight frits parts (w.f.p.) were added.
It was established that the highest enamel adherence with steel (Hmax) at 900 °C was reached at 0,012 g-mol Co₂O₃/100 w.f.p. (2 % CoO), 0,015 g-mol Ni₂O₃/100 w.f.p.(2,48 % NiO) and 0,045 g-mol CuO/100w.f.p. (3,6 % CuO) - Figure 1. It matched an optimum value of enamel melt conductivity at 900 °C, which were æ_opt = 0,210 - 0,215 Ω^-1 cm^-1

figure 1 - Dependence of enamels coatings adherence with steel from oxides containing: (1) = Co2O3; (2) = Ni2O3; (3) = CuO

figure 2 - Dependence of ground melts conductivity from adherence oxide containing: (1) = Co2O3; (2) = Ni2O3; (3) = CuO

Here the corrosion of the previously polished steel surface had a selective character, especially along grain boundaries. This was revealed by metallographic investigations.
Corrosion activity of enamel melts were not enough at æ < æ_opt and less than normal content of cobalt and nickel oxides (figure 2) or, on the contrary high development of corrosion processes in interface of copper - containing meltsmetal at æ> æ_opt lead to decreasing of H values. In the last case a similar character of steel roughening was caused by the effect of too intensive surface attack.
This was shown not only by microphotos, but steel oxidation values under coatings (figure 3) as well and especially changing of steel rod diameter (Δd, %) at æop_optr the best cobalt-containing ground Δd 28,87 %, for the best nickel-containing ground Δd = 28,5 %, but for coppercontaining ground at æ which æ>æopt_opt = 41,375%.
The data obtained explained the futile attempts to create ground coats which contained more electropositive cations than Co and Ni produce with the normal quantity of these metals i.e 1,5 - 2,0 %. This fact was the main objection to the electrochemical theory.
We established that enamel melts, containing CuO in the mentioned quantity, were characterized by increases of more than 0,3 Ω^-1cm^-1 conductivity due to the predominance of mobile cations Cu⁺. Only with CuO increasing up to 3,5 - 4,5 % , æ decreased to the required value because of one-valence cations Cu⁺ crossing in less mobile electrical current transfer Cu⁺. It was the predominance of these cations in enamels with the above mentioned CuO quantity that was found by investigation of their paramagnetic properties.

figure 3 - Dependence of steel 08KII oxidation from adhesion promoting oxides: (1) Co2O3; (2) Ni2O3; (3) CuO

As the result of these experiments we created Cu-containing ground coats for traditional enamelling technology in 1970s which were patented in a number of European countries (11) and Japan.
They also showed, that conductivity of silicate melts, unlike solution conductivity, played an essential role in steel corrosion at high temperature process development.
Probably it is connected with silicate melts structure peculiarities. The structure of these melts influences conductivity and diffusion. In this case an identical dependence of conductivity and diffusion oxygen- depolarizer of cathode processes by average ground coat melts investigated on structural factors were provided by approximate values of their activation energy: E_æ = 50-72 kJ/mol and ED = 50-80 kJ/mol.
Thus, increasing values æ of melts was followed by oxygen diffusion intensification and therefore growth of corrosion current in short-circuiting - microelements on steel surface in the process of ground-coat firing.
We kept to the parameters of æ_opt = 0,210- 0,215 Ω^-1 cm^-1 when we synthesised non-cobalt ground coat enamels for Puesta and especially Combismalt.
It was difficult to maintain a high value of specific resistance ρ_v at 20 °C in powders for electrostatic spraying.

Electrical properties of glass powders
The value of specific electrical resistance is the most important technological parameter of ground glass powders because it determines their chargeability and their adhesion to the metal substrate. It is known that low-melting glass powders, used for "two coat-one fire", have their individual resistivity (ρv ≤ 108 Ωm).
The necessary values of this property are obtained by means of special organic polymeric films with ρv>1014 Ωm from chlorsilane group. However, even when these films are used, very few glasses are suitable for electrostatic spraying.
The possibility of producing glass powders with ρv > 108 Ωm in pseudo-ternary system Cabal - R2O - SiO2 ha₂s been considered.
As it is known, Cabal glasses in the system C₂O-Al₂O₃-SiO₂ have extremely high specific volume resistance (12). According to Bruntsch and Hülsenberg (13) the region of glass formation in this system occupies the boron rich part of the diagram. However, according to these authors, the glasses formed have limited wetting power. SiO₂ as the second glass forming ingredient and alkali oxides are added in Cabal glass in order to improve this and other rheological properties.
The region of glass formation in the part of pseudo-ternary system Cabal-Na₂OSiO₂ limited by 70 mol% SiO₂ and 20 mol% Na₂O has been studied. These limitations are caused by the known effect of the formentioned oxides on increase in glass viscosity (SiO₂) and electric conductivity (Na₂O). The glasses (Table 1) were prepared from the appropriate analytical grade component and were melted in alumina crucibles between the temperature of 1200 and 1250 °C in the electrical furnace.

The glasses were ground to a grain size from 3 to 70 μm. Electrical resistance of powders without encapsulants was determined at heating and cooling 20-150-20 °C with application of the standard method.
It should be noted that a certain contribution to surface conductivity might be at the room temperature because of significant specific powder surface and absence of encapsulant. So values ρv obtained in the current work are not absolute quantities.
For comparison, analysis of model glass powder quality for dry electrostatic spraying resistivity data obtained when cooling from 150 °C to 20 °C were used. Steady increase in ρ of powders in the cooling regime (figure 4) indicates that the influence of their water humidity on conductivity became insignificant. One can see that there is a linear dependency of log ρ =f(T) for the glasses researched. As a result of ion nature of conductivity for the glasses researched,
lower glass powder resistance regular for glass powders belonging to compounds no. 3, 6, 8, 10 which contain 20 mol% Na₂O (figure 4 B ) as compared with no. 2, 4, 5, 7, which contain 10 mol% Na₂O (figure 4 A ).

A	B
figure 4 - Temperature dependence of the resistivity of glass powders in the pseudo-ternary system Cabal-Na2O-SiO2: (A) Na2O - 10 mol%; (B) Na2O-20 mol%

Also electrical resistance of powders decreased during gradual replacement of the Cabal-component with SiO₂ (with constant content of Na₂O). This proves the domain role of B₂O₃ as Cabal- component in this process. The value of conductivity activity energy were within the range from 0,839 eV to 1,567 eV for the glasses of this series. The possibility of increase in resistance of glasses no. 6, 7, 8 powders with reduced content of Cabal component by means of mixed alkali effect has been studied. The content of glasses with equimolar ratio of Na₂O and K₂O are shown in Table 2.

The temperature dependence of the resistance of glasses nos. 12, 13, 14 powders is shown in figure 5.

A	B
figure 5 - Temperature dependence of the resistance of glass powder in the pseudo- ternary system Cabal-R2O-SiO2: (A) R2O 10 mol%; (B) 20 mol%

It has been found that ρv values increased significantly, especially as far as glasses with 20% R2₂O (Olmost in two orders) were concerned, while the composition with 10% R₂₂O had its resistance increased by one order.
Thanks to the mixed alkali effect the resistance of glass powders which contain as little as 30 or 40 mol% Cabal and 20 mol % R₂O has increased to the level of resistance of glasses with 50-60 mol% Cabal and 10 mol % R₂O.
Compositions of ground coat copper containing enamels and cover coat enamels have been developed with glass no. 13 as a base and utilising the data about relation of melt conductivity, their interaction with steel and adherence strength of glass coating with steel.
It has been found that values ρ_vv of these enamels powders without encapsulants are 3x10⁹ Ωm for ground coat enamels and 5x1010 for cover coat enamels at room temperature.
This permitted a reduction of the electrostatic field voltage to 45-70 kV during application and provides good adhesion to the steel substrate of a powder which does not contain expensive constituents.
In the industrial process, optimum parameters for powders obtained were established. Adhesion (A, %), grain size (RF, %) and electric field voltage (U, kV) were checked.
RF characterizes quantity of particles more than 63 μm. Practically, it is very easy to adjust U.
Mathematical processing of experimental data with due regard for possible limits of values ρv = (10¹¹….10¹³) Ω m and RF (2,5 - 6) % at the required values A = (70-90) % gave the following dependence:

U = 2,73∗10³∗^0,13A∗RF^0,90,9∗ρ^-0,04

RF value limit from 2,5 to 6 % was established earlier when studying the relationship between specific powder surface, their humidity and ρ. The temperature dependence of the wetting (θ) and melting characteristics of both ground and cover coat enamels for "2 C / 1 F" have been studied. In addition to the dependence of the significance of θ and the softening temperature, the characteristics of the melting changes for each type of enamel were established. The configuration of the specific curves are analogous for Puesta and Combismalt: for example, the steep slope of first curve (Δt = 90-100 °C) up t = 630-650 °C and the gentle slope of the second curve (Δt = 150-170 °C) up t = 700-750 °C.
The influence of different de-setting and set-up agents on the rheological properties of slips for Combismalt were investigated.
Maximum exception of gas phase formation at coatings firing was important.
In connection with usage of steel 0,8KII containing C = 0,06 - 0,07 % in the Ukraine it was investigated that the usage of sodium phosphate as an desetting
agent was the most successful.
Industrial tests and developments of these frits were made at "Trade House Emal'zavod", Kharkov.

Conclusion
Research has developed principles of ground coat enamel synthesis whose melts are characterized by certain optimum values of specific conductivity (æ) at the melting temperature, irrespective of the added adherence promoter type.
The composition of a glass-matrix with a controlled structure strength for synthesis of low temperature frits having, at the same time, a high electrical resistance has been optimised.
The mathematical inter-dependence between electric field voltage, frit powder adhesion, its ρ_v and rinding fineness has been established, and copper-containing coatings with high adherence strength for Puesta and Combismalt have been produced.

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